Catalytic converters or mufflers have been used for some time to lower the noxious constituents in exhaust gas from internal combustion engines. Such converters function by passing the exhaust gas through a catalyst bed contained within the converter. The catalyst functions by catalyzing the oxidation of the hydrocarbon and carbon monoxide constituents and the reduction or disassociation of nitrogen oxides. Frequently additional air is mixed with the exhaust gas to aid in the oxidation reactions.
By-pass means have also been used in conjunction with such a catalytic converter to provide an alternate exhaust route around the catalyst bed. The by-pass has generally been controlled by a valve responsive to either exhaust backpressure caused by a plugged catalyst bed or excessively high catalyst temperature which results when a cylinder misfires.
One of the problems associated with catalytic converters is that their useful life is not as long as required to be of practical use in reducing air pollution. Degradation of the catalyst has been attributed to many things including additives normally included in blended gasoline. Such additives include lead antiknocks and phosphate esters. Even the additives placed in lubricating oil have been recognized as a source of possible catalyst degradation. These additives include such things as the zinc dialkyldithiophosphates which perform such a useful function in extending the useful life of the lubricant and in providing good engine lubrication. Even without any of the above additives present in either the fuel or lubricant, a practical catalyst has not yet been developed which will maintain its effectiveness during use over long periods of time (e.g., 50,000 miles). If the catalyst does not become deactivated by contamination it tends to break up and crumble during long periods of use due to thermal and other effects.